Re: [PATCH] arm64: mm: Fix kexec failure after pte_mkwrite_novma() change

["Chang, Jianpeng (CN)" <Jianpeng.Chang.CN@windriver.com>]

On 11/27/2025 1:41 PM, Anshuman Khandual wrote:
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> On 27/11/25 9:13 AM, Jianpeng Chang wrote:
>> Commit 143937ca51cc ("arm64, mm: avoid always making PTE dirty in
>> pte_mkwrite()") modified pte_mkwrite_novma() to only clear PTE_RDONLY
>> when the page is already dirty (PTE_DIRTY is set). While this optimization
>> prevents unnecessary dirty page marking in normal memory management paths,
>> it breaks kexec on some platforms like NXP LS1043.
>>
>> The issue occurs in the kexec code path:
>> 1. machine_kexec_post_load() calls trans_pgd_create_copy() to create a
>>     writable copy of the linear mapping
>> 2. _copy_pte() calls pte_mkwrite_novma() to ensure all pages in the copy
>>     are writable for the new kernel image copying
>> 3. With the new logic, clean pages (without PTE_DIRTY) remain read-only
>> 4. When kexec tries to copy the new kernel image through the linear
>>     mapping, it fails on read-only pages, causing the system to hang
>>     after "Bye!"
>>
>> The same issue affects hibernation which uses the same trans_pgd code path.
>>
>> Fix this by explicitly clearing PTE_RDONLY in _copy_pte() for both
>> kexec and hibernation, ensuring all pages in the temporary mapping are
>> writable regardless of their dirty state. This preserves the original
>> commit's optimization for normal memory management while fixing the
>> kexec/hibernation regression.
>>
>> Fixes: 143937ca51cc ("arm64, mm: avoid always making PTE dirty in pte_mkwrite()")
>> Signed-off-by: Jianpeng Chang <jianpeng.chang.cn@windriver.com>
>> ---
>>   arch/arm64/mm/trans_pgd.c | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
>> index 18543b603c77..ad4e5e4fcc91 100644
>> --- a/arch/arm64/mm/trans_pgd.c
>> +++ b/arch/arm64/mm/trans_pgd.c
>> @@ -40,8 +40,13 @@ static void _copy_pte(pte_t *dst_ptep, pte_t *src_ptep, unsigned long addr)
>>                 * Resume will overwrite areas that may be marked
>>                 * read only (code, rodata). Clear the RDONLY bit from
>>                 * the temporary mappings we use during restore.
>> +              *
>> +              * For kexec/hibernation, we need writable access regardless
>> +              * of the page's dirty state, so force clear PTE_RDONLY.
>>                 */
> Hence pte_mkwrite_novma() helper cannot be used here as would have
> normally expected. Might be good idea to add to the above comment.
I'll add this to make it clear that we can't use pte_mkwrite_novma().
>
>> -             __set_pte(dst_ptep, pte_mkwrite_novma(pte));
>> +             pte = set_pte_bit(pte, __pgprot(PTE_WRITE));
>> +             pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY));
>> +             __set_pte(dst_ptep, pte);
>>        } else if (!pte_none(pte)) {
>>                /*
>>                 * debug_pagealloc will removed the PTE_VALID bit if
>> @@ -57,7 +62,10 @@ static void _copy_pte(pte_t *dst_ptep, pte_t *src_ptep, unsigned long addr)
>>                 */
>>                BUG_ON(!pfn_valid(pte_pfn(pte)));
>>
>> -             __set_pte(dst_ptep, pte_mkvalid(pte_mkwrite_novma(pte)));
>> +             pte = pte_mkvalid(pte);
> Probably better to move pte_mkvalid() just after clearing PTE_RDONLY.
I'll make this change.
>
>> +             pte = set_pte_bit(pte, __pgprot(PTE_WRITE));
>> +             pte = clear_pte_bit(pte, __pgprot(PTE_RDONLY));
>> +             __set_pte(dst_ptep, pte);
>>        }
>>   }
>>
> Just wondering if it would be worth adding a local helper for the set
> PTE_WRITE --> clear PTR_RDONLY sequence describing its difference with
> now updated pte_mkwrite_novma() helper with the earlier comment.

Thank you for the review! I appreciate your suggestions.

You're right that a local helper would make the code more readable and 
clearly

document the difference from pte_mkwrite_novma().

I have a small concern about placing PTE manipulation functions outside of

pgtable.h - is this a good way? Or would you prefer this as a local 
static inline

helper within trans_pgd.c given its specific use case for kexec/hibernation?


I'll implement whichever approach you think is more appropriate in v2.


Best regards,

Jianpeng